Cheltenham Science Festival: The Kuiper Belt

Credit: CalTech

An artist's concept of Kuiper Belt Object Eris and its moon Dysnomia.

We caught up with Professors David Jewitt and Jane Luu prior to their lecture on the Kuiper Belt at this year's Cheltenham Science Festival. The pair discovered the belt in 1992 after committing their research to a region of space that had since then been considered completely empty.

How did you discover the belt and why were you looking for it?

J: Around 1987 Dave started wandering why the outer Solar System was so empty. Inside Jupiter there’s lots and lots of stuff, but beyond Jupiter there’s just a few big planets, Pluto then nothing else. So he thought, "Let’s do a search of the outer Solar System and see what’s happening."

D: Well, it was a good question. There’s nothing beyond Saturn except Uranus, Neptune and Pluto but why would that be? At that time nobody had computers powerful enough to examine the stability of orbits, so it was possible that all the objects beyond Saturn had been ejected by the massive gravity of the planets, so it could really be empty. Or, it could be that objects were just so faint that nobody had noticed them. They were the two possibilities and either one could be true.

After six years of searching using a CCD camera on the University of Hawaii's 2.24m telescope at Mauna Kea the pair finally discovered their first Kupier Belt Object (KBO).

Why had people not been looking there before?

D: It’s called out of sight out of mind. Astronomers are interested, as a group, in things that are far away. There’s a kind of prestige that the higher the red-shift (the greater the distance) the more important it is. People who work in the Solar System have exactly the opposite problem. They want to study objects as close as possible because they want to send out spacecraft. So in between, nobody cares, it’s just this region that nobody thought about. If there’s nothing there to think about why would you even try?

So it was perceived that this region was empty?

J: That’s right. Back in the fifties there were two guys, Gerard Kuiper and Kenneth Edgeworth, who thought a little bit about the problem. They thought that the solar nebula, where everything in our Solar System came from, was unlikely to have a sharp edge like our planetary system. If the solar nebula didn’t have a sharp edge it was likely that there were things beyond the giant planets. A swarm of smaller comet like bodies beyond the planets that never had time to grow very big. But nobody paid them much attention because there was no model, it was just a speculation.

D: If you read the paper it’s just a paragraph. Kuiper went even further, he was wrong about Pluto, he thought it was an Earth mass body, so he said, "Well look, if you’ve got this massive planet out there it’s going to scatter away all the objects around it so there should be nothing in this region." So the thing that is now called the Kuiper Belt was predicted by Kuiper to be empty. He anti-predicted the belt.

Since your discovery Pluto has been downgraded to a dwarf planet. What are your thoughts on the International Astronomical Union's planetary categoristation guidelines?

J: That was rubbish. The silliest thing is that there was a vote, how do you vote on a scientific fact? We don’t vote on the gravitational constant. But now Pluto has been downgraded to a dwarf planet, it’s a wishy washy thing because the planet is still there but it’s not really a planet.They want to please everybody. They don’t want to take away the title "planet" from Pluto because a lot of people study it and they’re sending spacecraft there - it would look really bad to go to an ex-planet. .

J: People have the perception that a planet is just a more impressive body, if you have a mission to go to a planet it sounds better than if your going to say a comet, it just sounds better.

Do you have any involvement with NASA’s New Horizons mission?

D: Zero.

Do you think there will be any good results?

D: No. There will be pretty pictures. It’s going to look like Triton. It’s bigger than Pluto, another KBO captured by Neptune as a satellite, and we have pictures of that. They’re not that good, maybe we’ll get some better pictures from the spacecraft when it goes by Pluto.

J: Did they get much data on Triton?

D: You can see stuff on the surface you can see ice, you can see craters, you can see atmospheric phenomenon. But I don’t see why Pluto would be any different. We’ll get some science from the mission but it won’t actually change our picture of the Kuiper Belt at all because it’s one object. How can we change our perception of the whole belt by looking at one object? You’ll find out details about one object but the big picture remains the same. And the big picture is so far measured by telescopes on the ground because that’s the only way you can see a lot of objects, you can't send a mission to every one.

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